Michael A. Reshchikov

Luminescence properties of defects in GaN

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

Dislocation density in GaN determined by photoelectrochemical and hot-wet etching

Defects in GaN layers grown by hydride vapor-phase epitaxy have been investigated by photoelectrochemical (PEC) etching, and by wet etching in hot H3PO4 acid and molten potassium hydroxide (KOH). Threading vertical wires (i.e., whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic-force microscopy, we find the density of “whisker-like” features to be 2×109 cm−2, the same value found for the etch-pit density on samples etched with both H3PO4 and molten KOH. This value is comparable to the dislocation density obtained in similar samples with tunneling electron microscopy, and is also consistent with the results of Youtsey and co-workers [Appl. Phys. Lett. 73, 797 (1998); 74, 3537 (1999)].

Energy band bowing parameter in AlxGa1−xN alloys

Molecular-beam epitaxy grown AlxGa1−xN alloys covering the entire range of alloy compositions, 0⩽x⩽1, have been used to determine the alloy band gap dependence on its composition. The Al chemical composition was deduced from secondary ion mass spectroscopy and Rutherford backscattering. The composition was also inferred from x-ray diffraction. The band gap of the alloy was extracted from low temperature optical reflectance measurements which are relatively more accurate than photoluminescence. Fitting of the band gap data resulted in a bowing parameter of b=1.0 eV over the entire composition range. The improved accuracy of the composition and band gap determination and the largest range of the Al composition over which our study has been conducted increase our confidence in this bowing parameter.

Dependence of GaN polarity on the parameters of the buffer layer grown by molecular beam epitaxy

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.

GaN epitaxy on thermally treated c-plane bulk ZnO substrates with O and Zn faces

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga2ZnO4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O- and Zn-face ZnO.

Two charge states of dominant acceptor in unintentionally doped GaN: Evidence from photoluminescence study

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence (YL) band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence (GL) band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex.

Yellow and green luminescence in a freestanding GaN template

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving gallium vacancies and the YL to the same defect, but bound to dislocations, or possibly to structural surface defects.

Photoadsorption and photodesorption for GaN

The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UV illumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaN surface. In contrast, the upward band bending gradually decreases in vacuum under UV illumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration.

Surface band bending in as-grown and plasma-treated n-type GaN films using surface potential electric force microscopy

The surface band bending, as well as the effect of plasma-induced damage on band bending, on GaN surfaces, was investigated. The upward band bending, measured by surface potential electric force microscopy (a variant of atomic force microscopy), for the as-grown n-type GaN was about 1.0 eV which increased to ∼1.4 eV after reactive ion etching (RIE). UV illumination decreased the band bending by 0.3 eV with time constants on the order of seconds and hundreds of seconds for the as-grown and RIE treated GaN, respectively. This implies that there is a higher density of the surface states in the samples subjected to the RIE process. After the RIE treatment, the shape of the photoluminescence spectrum remained unchanged, but the intensity dropped by a factor of 3. This effect can be attributed to nonradiative defects created near the surface by the RIE treatment.

Photoluminescence of GaN grown by molecular-beam epitaxy on a freestanding GaN template

Photoluminescence (PL) studies were performed on a 1.5-μm-thick GaN layer grown by molecular-beam epitaxy on a freestanding GaN template that in turn was grown by hydride vapor-phase epitaxy. PL spectra from both the epilayer and the substrate contain a plethora of sharp peaks related to excitonic transitions. We identified the main peaks in the PL spectrum. Taking advantage of the observation of donor bound exciton peaks and their associated two-electron satellites, we have determined the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively.